Haven't built a bow in 2 years now

[Pappy]

Can't wait to see that one,I feel sure you will reach your gold.I don't build as many either,involved in a lot but not building them myself,I have a lot of other things that take away from my bow building time,it's all good,when working with your hand and learning new things. 
   Pappy

[Jude]

I just read your post, and realized that its been over 2 years for me as well.  (I used to post as Brokestick, but that account got deleted when the forum migrated and I wasn't paying attention)  Seems like a succession of Army schools, broken down tanks and weapons when I wasn't in school, a move from CA to NY, and another deployment have all conspired to make me lose track of time.  It seems, too, that I cycle through interests periodically.  Unfortunately, the archery bug came back right before this deployment, but I had enough time to get my kids going on a couple store-bought bows, and a toy hickory bow (10# @ 20"), that was the last thing I made, for my youngest to learn on.  I have 2 unfinished bows for my older boys from the same period, but by the time I get to finishing them, I'll have to make a bigger one for my oldest, and move those 2 down the line. 

As for the siege engine you're planning (it would be a ballista), I believe medieval engineers went early on with rigid limbs and rope torsion devices, allowing them to adjust tension, and likely increasing the safety factor, as well as, decreasing the dimensions.  check out this site:   www.siege-engine.com

[Jude]

OOPS, Guess I should have read the whole thread before I replied.  This isn't your first foray into the siege engine realm.  Best of luck.

[Badger]

Jude, good point you made about using torsion devices to decrease demensions. Attempting to use wood to power these giants was one of DaVincis failings I believe. Wood will just not store the needed energy for the heavy projectiles they were using. I will probably need about 15,000# stored energy for this project I am working on. I could easily get this with a much smaller machine using torsion. Steve

[Christophero]

I remember when you mentioned you may slack off the bow building sometime back.  Looks like you were true to your word!
I haven't built a bow since late July when I built a hickory bow backed with Marine digital camo for my son who just graduated boot camp. 
It shot like a house on fire but then just toward the stiff tip the limb stated to chrysal and give.  Unless I try to patch it the stick has become a
wallhanger.  I've been frustrated by how many bows I've made and so few of them I am really content with. 
The all wood bow is still in my blood but presently I am finishing up a recurve form to give a fb lam bow a shot.  At least, if this works out, I will have a bow
that will last a lifetime (?) to shoot while I perfect my self and sinew backed bow craft.

[Tom Leemans]

Badger,
They once tried to build DaVinci's giant cross bow on TV. They kept breaking it. Of course, I don't think they followed what was a mediocre design in the first place. The "limbs" just weren't designed to bend as intended. The one time they got it to shoot, it was the biggest dog I ever saw.

[Jude]

Another thing came to mind this morning.  Things don't scale up or down as we think they should, because of the relationship of surface area to volume.  if you make an object twice as big, it becomes 8 times as heavy.  If you only look at it in 2 dimensions, like when you calculate beam strength,  doubling thickness increases strength (ie:  resistance to bending) is increased 4 times.  Beams work the same as bow limbs; only the top and bottom surfaces do all the work, and the wood in the middle is there as a spacer.  Increase the distance between the working surfaces, and the stiffness/strength increases by a factor of 2.  If I was to double the size of a long bow, I believe it would work as follows:  start with a 6' x 1" bend in the handle yew long bow 50# @ 28".  Double the width=100# @ 28".  Double the thickness=400# @ 28".  Double the length, and the stiffness should drop by half to 200# @ 28", but should come back up to 400# @ 56".  I'm not an engineer, so I'm a little scetchy on that last part.  If you were to look at the bow as the limbs work, at full draw, each individual inch of wood is only bending half as much as it did in the shorter bow, but since it is 4 times as stiff, the back and belly are still under twice the stress from tension and compression.  The question is, can the wood handle it?  The answer is, probably not.  To scale up a bow, it has to become proportionally wider and thinner than the original, and there's an upward limit on the practicality of that.  I think my double size bow, if I wanted only to double the draw length and weight, would stay the same thickness as the original, and become a flat bow.  Even if I were to only make it 1.5" wide, I would only make it 1.15 times as thick to make it 200# @ 56".  It reminds me of when I made the toy bow for my little guy,  It is only slightly shorter than an adult bow, slightly narrower, and about half as thick.  I was surprized when it only came out to 10# @ 20", but the scaling factor had worked in the other direction for me.  On the flip side, the little bow can be drawn to an adult draw length and has only taken 3/4" of set, under intense abuse.

[Dane]

Steve, you may want to spend some time looking at this guy’s blog. http://www.wattsunique.com/blog/ He is building a late Roman iron-framed catapult. It is an in-swinger, which is still a fairly controversial subject within the catapult community. I personally think Watts is taking torsion engineering to heights rarely if even seen, including the work of the Englishman Alan Wilkins. Maybe his work can give you some insights into ancient artillery in general, and specialized engineering required for these kinds of projects. If you have worked at all with catapults, you will find it is a very different discipline from tension power.

Also, you probably should strongly consider getting the two-volume Eric Marsden works. They are out of print and somewhat rare, but you can find a set for 150 – 200 dollars from rare book sellers. While a bit out of date at this point (he died in the early 1970s, and wasn’t around to analyze the newer archeological finds such as Hatra), he gives some good insights into large bow-powered machines. The Greeks tended to call all tension machines gastrophets, even after ratchets had been incorporated in larger machines. Going from memory (books are at home right now), Marsden believed that torsion power eclipsed tension power (giant bows) probably because bows of that size had reached their zenith with the Greeks.

These large machines can be dangerous. There are written accounts of crew members being decapitated when a component fails. A 50’ bow I imagine can be a very dangerous thing to be around if catastrophic failure occurs.

[Badger]

Dane, the tension powered machines are not as effective as the torsion models or even the dead weight models. This is part of what I want to demonstrate. Kind of a slap in the face to Da Vinci. The show was very disapointed when I pointed that these huge machines we were building would only be capable of shooting very light weights like maybe 2#. Da Vincis drawing indicate bending wood springs in many configurations. They have unworkable string angles and on the average are about 4 times thicker than would be even remotely possible. I spent the last couple of years building scale models and then modifying them to work. The torrsion machines in those days would throw buolders and dead horses. These tension machines if properly tuned could only throw small bombs and rocks at best up to about 10# for a 100 yard toss. When we shoot a bow and arrow the arrow is only about 1/1000 ratio of stored energy to payload for a speed of about 170 fps. I will be looking for about 400 fps so will be looking for a ratio of about 5,000/1. It will be almost dry fire. Most of my wood bows break when they hit the 280fps range using light arrows. I am also working on a sling shot using wood torsion springs and cables. I will post some photos tomorrow or later today. Wood can generate decent speeds but is not good for heavy payloads. Steve

[Dane]

I am no Da Vinci scholar, but my feelings are that many, many of his designs are flights of fancy, or were done to satisfied patrons and gain employment. As a military weapons engineer, he was not particularly successful. Blasphemy eh?

I find it particularly interesting that the technologies developed in the Classical period were lost and bastardized / reinvented in the medieval and Renaissance periods, and were never particularly successful. Gunpowder of course was coming into its own when Da Vinci was drawing his drawings, and would doom earlier military technologies such as catapults, bows, and so on. A handgonne could be forged by a competent smith in a few hours, was very cheap, and does not need the years (lifetime) of training an archer does. A peasant or ordinary solider could be taught to load and fire a handgonne in a very short period of time, and his cost to the king about to start a campaign or war vs. a knight or bowman was very cost-effective. It also defeats the finest plate armor available. More efficient and cheaper ways to kill your enemy is what engineers were after, and that still continues.

I find it kind of funny and ironic that as you lay dying on some battlefield, your guts falling between your hands, you should blame some guy with a slide ruler for your demise.

This is a personal belief, but weapons design is mentally different than bow design, even the “war bows”. I keep in mind the sole purpose of these machines, which is efficient killing at a distance. Warfare is brutal and stark, and these machines only exist for war. A target for a bow can be a bale of hay for recreational shooting or a deer or elk, while a target for a catapult is always going to be a human being.

Bolts / arrows from these machines are quite a bit smaller than those needed for bows, as well. Maybe a giant bow needs a very small projectile, too. In that show Surviving History, they build a torsion catapult and did a good job, but they were shooting broomstick length spears, which made a lot of that work moot. The spears kind of sailed out of the machine and did interesting things trajectory-wise, if my memory serves me.

The guy Watts has been getting 350+ fps or so for his machine, which are impressive numbers, and point to what can be done with a really large machine. The big palitone stone throwing ballistas, most likely could be adapted to fire arrows, as well.

Dane

[Badger]

  Dane, I went to that web sight. He is doing some good research over there. Very interesting stuff. All the best machines in those days were built by backyard engineers with lots of common sense. About the same is true for the bows and arrows we build today. Steve

[Jude]

It just occurred to me that the whole scale dilemma works in the opposite way for firearms and artillery.  When you scale up a cartridge, you increase the powder charge by the same amount as the projectile weight, and the surface area to volume change works in your favor, when it comes to wind resistance.  A 5.56mm NATO and a Cal, 0.50 BMG are nearly scale models of each other, and leave the muzzle at nearly the same velocity, but the 50 Cal travels twice as far.  Bigger is better with artillery, until it gets too heavy to move around conveniently, but it is much easier to make small bows than to make large ones.  The larger the bow, the fewer choices you have in suitable wood.

[Dane]

  Dane, I went to that web sight. He is doing some good research over there. Very interesting stuff. All the best machines in those days were built by backyard engineers with lots of common sense. About the same is true for the bows and arrows we build today. Steve

Glad you liked what was on the site. A nice guy too, lives in Washington State, so not too far from you.

Dane

[Tom Leemans]

As usual though, you start with an idea, build it, see what worked and what failed, then decide whether you can improve it, or start over. Improvements are made until you have a big pile of crap hiding out back, and a success story in your hands to show everyone. Then people call you a genius!

[Badger]

Jude, theoreticaly with bows each time you double the size you 8 times the payload and 8 times the stored energy and double the dstance, don't quote me on that but i think I saw that when researching somewhere. I plan to start with a 12 ft bow then a 24 ft bow and these should tell me a lot about how it will scale up. Going to use a simple pyramid design to avoid tillering, will also allow some slack in the design instead of trying to peak it out right away. More intertested in the scaling process than peak performance at this point

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